AVI poses a question:
When did Europe become Christian? Arguably never, he says.
If you want to debate that topic, please do it at his place because he deserves to enjoy the discussion there. What I want to do here is discuss a model that explains just why we can't really answer that question in a fully satisfactory way. I think it's a good model to have in your mind for a lot of purposes, one that many of you will find helpful.
The model belongs to Timothy Williamson. He came up with the basic approach in his work on
vagueness, but later realized that it could serve as a revolutionary model for epistemology (which, I assume everyone knows, is defined as 'the study of knowledge,' but really is mostly a 2,000+ year debate about what exactly knowledge might be).* He wrote a book called
Knowledge and its Limits that explains the epistemic model. It's become a big hit in the philosophy world because everyone hates the idea, but it's hard to show exactly where he goes wrong (if indeed he does).
The basic idea works like this: you can know things, and you can sometimes also
know that you know them. Other times, however, you are close enough to a border such that you can't really be sure that you know what you know (so that you know, but don't know that you know). Accepting this explains both vagueness and why we sometimes can't be sure about question's like AVI's.
The example on vagueness is also an example about knowledge, so I'll just borrow it. Say that on a given day, day
n, you are a child and you know that you are a child. Childhood lasts a long time, so presumably tomorrow (day
n+1) you will also be a child. Since there is no obvious limit on that, you should remain a child forever: but somehow a day comes, say by day
n+10,950, are not a child and you know that you are not a child anymore. Clarity exists on both ends of the spectrum.
So which day was the exact day on which you stopped being a child? There wasn't one, of course; somehow it happened, during a period of time in which you weren't really sure anymore. Sometimes you felt like a child, sometimes you could see yourself taking adult steps and becoming more adult as a consequence. Exactly when it happens is not clear.
Williamson's answer, in other words, is to dispose of certainty and embrace vagueness. I'm cold at 32 degrees F, and I know it; I'm warm at 75 degrees, and I know I'm not cold anymore. But as the temperature rises, there might come a point that even with careful reflection I couldn't say whether I was still cold. We could probably narrow that down with experiment, but it might vary a lot depending on weather conditions. A bright sunny windless day might no longer feel cold at 34, whereas a windy, wet, rainy day might feel quite cold even at 60 (hypothermia, in fact, is possible). But there will be a moment at which I'm plausibly not really sure.
That doesn't mean that we lose knowledge. We can not only know but
know that we know at the ends of the spectrum. We lose that second-order certainty as we get closer to the border conditions; we might
know but not
know that we know. Very close to the border, we might not know.
So when did Europe become Christian? If the answer really is 'arguably never,' then we still are close enough to the border condition that we can't say we know it ever did. But I think we could say that we
know that we know that European civilization was Christian in the 19th century. That seems like a flower of clarity. It may well be, as Eric Blair has often argued, that this civilization received its death blow in WWI and has been dying ever since. At some point we can't still say that we know that Europe is Christian at all, even if once we knew that it was and knew that we knew it.
* I'm leaving out a discussion of Williamson's argument that knowledge isn't analyzable, and focusing here on the vagueness aspect of knowledge, which I think is the more useful concept.